Use the Product Rule to Multiply $\sqrt { \frac { x } { 7 } } \cdot \sqrt { \frac { y } { 3 } }$

Multiple Choice

Use the product rule to multiply. Assume all variables represent positive real numbers.
- $\sqrt { \frac { x } { 7 } } \cdot \sqrt { \frac { y } { 3 } }$

A) $\sqrt { \frac { 3 x } { 7 y } }$
B) $\frac { x y } { 21 }$
C) $\sqrt { \frac { x y } { 21 } }$
D) $\sqrt { \frac { 3 x + 7 y } { 21 } }$